1. Field of the Invention
The invention relates generally to computer memory and more specifically to removable hard disk cartridges and disk drives.
2. Description of the Prior Art
The recording and reading of rotating magnetic hard disks has conventionally included the use of inductive read/write heads. However, as track densities have increased, conflicts have arisen between optimum read head design and optimum write head design. Higher recording frequencies require less inductance and therefore fewer coil turns in the head. The narrower recorded data tracks produce fainter pickup signals, so more sensitivity is required and this means more coil turns are needed in the head.
Magneto-resistive materials have recently been employed in magnetic disk recording read heads. Such materials experience a change in their resistance in the presence of magnetic flux. A change of two percent in the resistance is typical. However, the change in resistance is not linear and a magnetic bias is required to move the center point of operation into a small area that is linear. Such bias is typically provided by a small permanent magnet positioned nearby.
Magneto-resistive (MR) heads are easily saturated by ordinary disk recording levels and for this reason, hard disks recorded by ordinary inductive read/write (R/W) heads are not always compatible with MR head disk drives. In fixed disk drives this is not a problem because the read and write heads and hard disk are part of a permanent set. In removable hard disk drives this can be a significant problem area.
MR heads typically have narrow pickup widths. Within a comparatively widely-written data track, the narrow read width is not a problem. In fact, some extra degree of disk run out and track misregistration can be tolerated as the MR head can be allowed to wander within a data track radially without producing a concomitant read-amplitude variation.
Prior art embedded servos are a problem with MR heads because of the narrow read width. A typical prior art embedded servo consists of two bursts, an "A" burst and a "B" burst, that head a block on a data track. The "A" burst is offset radially one-half of a track width, and the "B" burst is offset radially in the opposite direction one-half of a track width and longitudinally behind the "A" burst. An inductive R/W head passing the "A" and "B" bursts while at center track will glance both bursts equally and will produce output pulses for each of equal amplitude. If the R/W head is off center, the pulses will not be equal in amplitude, and which one has the greater amplitude will indicate which radial direction the R/W head is from dead center. A servo system is conventionally employed to keep the R/W head at track center in real time, even if the recorded track is not perfectly circular or perfectly concentric with the disk hub. A typical MR head has a read width so narrow that the "A" or "B" servo bursts may not be seen at all as the head moves from track center by more than 25% of a track width. Further complicating the use of MR heads with embedded servo features is the fact that MR heads often do not present symmetrical responses, thus comparative "A" and "B" burst amplitudes are not necessarily representative of the MR head position between the bursts. In fact, a properly positioned MR head can fail to recognize its good position by responding differently to bursts that are on opposite sides and/or that have opposite polarities.
The asymmetry of MR element off-track performance can be adequately modeled as the area of the stripe of MR material subjected to a magnetic field as the element passes over an isolated track written with an inductive head. The magnetic field propagates through the MR stripe normal to the angle of MR bias. This angle of incidence along with a nearby permalloy shield casts a magnetic shadow over the element. Such a shadow causes the asymmetry in sideways reading that an MR element shows when it moves off track in opposite directions. Thus the degree of asymmetry is dependent on the MR stripe height and bias angle. Such subjects are discussed by A. Wallash, et al., in "Dependence of magneto-resistive head read back characteristics on sensor height", published in the Journal of Applied Physics, 69(8), Apr. 15, 1991.
Simply moving the "A" and "B" bursts such that they are offset radially from track center so that they can be detected by the MR head at a certain track offset is not completely satisfactory because the detection of other track offsets by the MR head will be compromised. The MR head can get lost in the blank area that would be created between adjacent tracks.
The prior art describes magneto-resistive read heads only in fixed hard disk drives. The problems that are encountered in removable hard disk applications have so far been left unaddressed, even by those advanced in the development of the art, e.g., IBM.